1. Field of the Invention
This invention relates to a dielectrically-loaded antenna and to a feed structure for such an antenna.
2. Discussion of the Related Art
British Patent Applications Nos. 2292638A and 2310543A disclose dielectrically-loaded antennas for operation at frequencies in excess of 200 MHz. Each antenna has two pairs of diametrically opposed helical antenna elements which are plated on a substantially cylindrical electrically insulative core made of a material having a relative dielectric constant greater than 5. The material of the core occupies the major part of the volume defined by the core outer surface. Extending through the core from one end face to an opposite end face is an axial bore containing a coaxial feed structure comprising an inner conductor surrounded by a shielded conductor. At one end of the core the feed structure conductors are connected to respective antenna elements which have associated connection portions adjacent the end of the bore. At the other end of the bore, the shield conductor is connected to a conductor which links the antenna elements and, in these examples, is in the form of a conductive sleeve encircling part of the core to form a balun. Each of the antenna elements terminates on a rim of the sleeve and each follows a respective helical path from its connection to the feed structure.
British Patent Application No. 2367429A discloses such an antenna in which the shield conductor is spaced from the wall of the bore, preferably by a tube of plastics material having a relative dielectric constant which is less than half of the relative dielectric constant of the solid material of the core.
Dielectrically-loaded loop antennas having a similar feed structure and balun arrangement are disclosed in GB2309592A, GB2338605A, GB2351850A and GB2346014A. Each of these antennas has the common characteristic of metallised conductor elements which are disposed about the core and which are top-fed from a feed structure passing through the core. The conductor elements define an interior volume occupied by the core and all surfaces of the core have metallised conductor elements. The balun provides common-mode isolation of the antenna elements from apparatus connected to the feeder structure, making the antenna especially suitable for small handheld devices.
The feed structure is formed in the above-noted antennas as follows. Firstly, a flanged connection bush, plated on its outer surface, is fitted to the core by being placed in the end of the bore where the feed connection is to be made. Then, an elongate tubular spacer is inserted into the bore from the other, bottom, end. Next, a coaxial line of predetermined characteristic impedance is trimmed to length and an exposed part of the inner conductor at one end is bent over into a U-shape. The formed section of coaxial cable is inserted into the bore and the elongate tubular spacer from above and the entire top connection is soldered in two soldering steps: (a) soldering of the inner conductor bent portion to connection portions of the antenna elements on the top face of the core, and (b) soldering of the flanged bush to the shield conductor and to further antenna element connection portions on the top face of the core. The core is then inverted and a second plated bush is fitted over the outer shield conductor of the cable where it is exposed at the opposite end of the core from the bent section of the inner conductor so as to abut the plated bottom end face of the core. Finally, this second bush is soldered to the outer shield conductor and to the plated bottom end face of the core.
One of the objectives in the design of the antennas disclosed in the prior applications is to achieve as near as possible a balanced source or load for the antenna elements. Although the balun sleeve generally serves to achieve such balance, some reactive imbalance may occur owing to constraints on the characteristic impedance of the coaxial feeder structure and on its length. Additional contributing factors are the difference in length between the inner and outer conductors of the feed structure, e.g., as a result of the bent-over part of the inner conductor, and the inherent asymmetry of a coaxial feed. Where necessary, a compensating reactive matching network in the form of a shorted stub has been connected to the inner conductor adjacent the bottom end face of the core, either as part of the device to which the antenna is connected or as a small shielded printed circuit board assembly attached to the bottom end face of the core.
The applicant's co-pending International Patent Application No. PCT/GB2006/002255 discloses an antenna feed structure and a method of assembling a dielectrically-loaded helical antenna. The feed structure comprises the combination of a length of coaxial transmission line and a laminate board extending laterally of the axis defined by the coaxial line. The board is secured to the distal end of the coaxial transmission line by a plurality of lugs, formed integrally with the coaxial outer conductor at its upper edge, the lugs passing through holes in the laminate board. During assembly of the antenna, the feed structure combination is inserted into the distal end of the antenna core. The board comprises two circular insulative layers and two conductive layers. One of the conductive layers is formed on a proximal surface of a proximal insulative layer, and the other conductive layer is sandwiched between the two insulative layers. When in position, the layers of the laminate board are arranged such that a shunt capacitance is formed across the inner and outer conductors of the coaxial transmission line and across at least one pair of helical antenna elements. The combination of the length of coaxial transmission line and the laminate board constitute a unitary feed structure which can be assembled prior to insertion into the antenna. In this way, the laminate board provides a matching structure between the antenna elements and the transmission line.
The shunt capacitance of the above-described laminate board structure is limited by the area of the layers, the depth of the insulative layers and the dielectric constant ∈r of the proximal insulative layer. In practice, this means that there is a lower limit to the frequency at which the laminate board can act as an effective matching structure. In particular, it has been noted by the applicant that although the design is suitable for a satellite radio operating around 2.3 GHz, the design cannot provide high enough capacitance to be effective for GPS L1-band signals at around 1.5 GHz.